1. Field of the Invention
The present invention relates to optical sensing apparatus.
Optical sensing apparatus, that is, sensing apparatus comprising one or more optical sensing elements, is useful for obtaining information on physical conditions within environments in which electromagnetic emissions are undesirable or dangerous. Optical sensing elements are robust and require little or no maintenance as they have no moving parts. For example a sensing element of a fibre-optic strain sensor consists only of a length of optical fibre; conditions external to the sensing element are inferred from changes in charateristics, such as amplitude, frequency or polarisation, in light received from fibre.
2. Discussion of Prior Art
International application PCT/CH99/00607 (international publication number WO 00/39552 discloses optical sensing apparatus having a sensing element consisting of a fibre laser. The fibre laser is birefringent to an extent which depends on a pressure difference experienced by different parts of the fibre laser. The birefringence, and hence the pressure difference, is inferred from beat frequencies between polarisation modes of the fibre-laser. The sensor system may be used to measure the flow rate of fluid; however this requires use of a Venturi tube, which adds to the complexity of the apparatus. Furthermore, the apparatus is not capable of additionally providing data relating to the temperature of the fluid.